The invention generally relates to methods for fabricating integrated circuits (ICs) and semiconductor devices and the resulting structures. More particularly, the invention relates to dummy wafers used in IC manufacturing and methods for making such structures. Even more particularly, the invention relates to dummy wafers with increased lifetimes and methods for making such dummy wafers.
Chemical vapor deposition (CVD) is one common method used to form the various films and layers that are used to make the components in an integrated circuit (IC). There are various types of CVD, such as low pressure (LP) CVD, high pressure (HP) CVD, plasma enhanced (PE) CVD, as well as others. CVD can be used to deposit many different types of materials, such as silicon, dielectric materials, and metals such as tungsten or titanium.
Silicon (Si) is one material that is often used in ICs. One method to deposit silicon in thin film form is by using LPCVD. In this method, wafers are heated in a pressure vessel to between 450xc2x0 C. and 650xc2x0 C. while atmospheric gases are pumped out. When the pressure in the vessel, such as a quartz tube, reaches 200-1000 mTorr and the wafers have reached such a desired temperature, a silicon containing gas such as silane (SiH4) is flowed into the vessel. Thermal breakdown of SiH4 results in the deposition of Si on the surface. Excellent control of the thickness of the film, the thickness uniformity across a wafer, and step coverage (the ability to cover varied topography on the wafer) is achieved by adjusting the operating parameters (like temperature, pressure, and gas flow) in the vessel.
Many IC manufacturing processes not only require the deposition of thin films of silicon, but also require that the thin film be doped to make it conductive. Doping processes introduce foreign atoms (ions)xe2x80x94like boron (B) or phosphorous (P)xe2x80x94into the silicon material to modify the conductivity of the Si atoms. One common method to dope silicon is by ion implantation, a process where foreign atoms are ionized, accelerated by electromagnetic fields, and then directed to impact (xe2x80x9cimplantxe2x80x9d) into the silicon. Ion implantation requires expensive equipment and does not permit good concentration depth profile control of the dopant.
Another doping method (in-situ doping) co-deposits the dopant atoms during the process used to deposit the silicon material. Gases such as boron trichloride (BCl3) or phosphine (PH3) are admitted to the chamber simultaneously with silane, resulting in boron- or phosphorus-doped silicon films. This method allows for better control of the depth profile concentration when compared to ion implantation. The concentration depth profile control is achieved by varying the gas flows with time. The cost is also less because silicon film deposition process and the doping process both use the same chamber and are performed simultaneously.
Numerous systems and types of apparatus are used for CVD. Generally, most CVD systems contain a reactor, gas-pumping system, an exhaust system, a temperature control system, and a process control system. For example, a tubular-type low-pressure chemical vapor deposition (LPCVD) system is shown FIG. 1. In this commonly used batch-type CVD system, the system 2 contains a vessel or reactor 10 made of annealed quartz. The reactor 10 is surrounded by a heater 12 used for heating the reactor 10. A boat 20 that carries a number of wafers 18 is then transported into the reactor 10 and placed at a pre-determined position. The exhaust gas of the LPCVD process is released by an exhaust system through a releasing end 16. The boat 20 further contains a wafer stage for holding a certain number of wafers 18, including both product wafers and dummy wafers spaced throughout boat 20. When a LPCVD process is performed, the gaseous reactant 22 is pumped into the reactor 10 from a valve 14.
When using this system 2, the tube, boat, and dummy wafers are all deposited with silicon along with the desired product wafers. The dummy wafers are non-product wafers that are used to maintain consistent deposition (film thickness uniformity) on the product wafers. For example, when a boat holds 150 wafers, it may desirable to deposit silicon on 50 wafers in one deposition and on 100 wafers in another deposition. However, the gas flow dynamics and surface area of 50 wafers loaded in the boat is different than for 100 wafers. Dummy wafers are used such that each deposition is done on 150 wafers, maintaining film thickness uniformity on the product wafers and consistency in the deposition process from one batch to another.
Dummy wafers are often used repeatedly for many depositions. Dummy wafers, however, break when the silicon thickness becomes excessive and must therefore be replaced on a regular basis. Breakage of dummy wafers is often fatal for the product wafers that accompany the dummy wafers in the boat. Particles coming from the broken wafers are spread throughout the system, contaminating product wafers. As well, the robot arm (not shown in FIG. 1) that unloads the dummy wafers may xe2x80x9cramxe2x80x9d the broken wafers, causing a moment arm to develop on the quartz boat holding the wafers, often resulting in the boat breaking and the entire load of wafers (both product and other dummy wafers) dropping and breaking.
For the reasons detailed above, as well as other reasons, dummy wafers are typically replaced before they break. Tests are done to determine the thickness of the silicon that can be deposited on the dummy wafer before they break, and then a replacement schedule is developed to replace the dummy wafers well before they break. However, replacing the dummy wafers means xe2x80x9cdown timexe2x80x9d for the CVD system while the dummy wafers are being replaced, e.g., no product wafers can be produced. As well, there is a significant cost in replacing the dummy wafers.
The present invention provides dummy wafers that are used in IC manufacturing and methods for manufacturing the same. The dummy wafers are made with an increased resistance to breaking during CVD manufacturing process. The dummy wafers are made by placing a protective film over the wafer surface(s) exposed during the CVD process. By increasing the resistance to breaking, the protective film extends the useful life of the dummy wafers.